WASHINGTON (AP) — A briefing paper prepared for Attorney General Michael Mukasey during the Bush administration in 2007 outlined failed attempts by federal agents to track illicitly purchased guns across the border into Mexico.It also stressed the need for U.S. and Mexican law enforcers to work together on such efforts using a tactic that's now generating controversy.The information contained in one paragraph of a lengthy Nov. 16, 2007, document marks the first known instance of an attorney general being given information about the tactic known as "gun-walking."It has become controversial amid a probe by congressional Republicans criticizing the Bureau of Alcohol, Tobacco, Firearms and Explosives for using it during the Obama administration in an arms-trafficking investigation called Operation Fast and Furious.The probe focused on several Phoenix-area gun shops.